Increased capillary permeability remains a critical determinant of morbidity and mortality among ICU patients. A primary component of the lung permeability barrier is the endothelial glycocalyx, a polymer scaffolding composed of glycoproteins and glycosaminoglycans on the cell surface and overlying the cell-cell junction. We have shown that components of the glycocalyx participate in agonist-mediated signaling, and flow- and pressure-mediated mechano-transduction, all of which result in barrier dysfunction. We hypothesize that the glycocalyx functions as both a molecular filter, which determines fluid and solute flux into the cell-cell junction, and as an active signaling interface between blood born agonists, hemodynamic force and the endothelial cell itself. This application proposes to examine the role of the glycocalyx on barrier function, using intact lungs and a cell-culture model, thus allowing for a direct comparison of the functional attributes of the glycocalyx in vivo vs. in vitro. In Specific Aim 1, we will characterize the role of the glycocalyx on lung capillary permeability in isolated, perfused lungs and in whole animal studies. These studies will examine the glycocalyx as a molecular filter, in agonist-mediated barrier dysfunction and in mechano-transduction. In Specific Aim2, we will characterize the role of specific glycoproteins in mechano-transduction and characterize the mechanistic pathway(s) involved. In Specific Aim 3, we will use sophisticated biophysical techniques to characterize the micro-biomechanical properties of the glycocalyx and its constituents that are fundamental to mechano-transduction. The integrated results of this application will provide a comprehensive understanding of the glycocalyx as an important interface between the capillary wall and the vascular compartment. We believe the results from these studies will provide entirely new information about the regulation of lung capillary barrier properties and could lead to the development of novel therapeutic interventions to attenuate the deleterious effects of increased lung permeability.